Prestressing a pipe string in a well cementing method



swan WM PRES'IRESSING A PIPE STRING IN A WELL CEMENTING METHOD FOLLOWER FLUID WITH ICE PARTICLES n H 3 n inc. n r I a o 6 n 0 0D inlfiia n.1 q G. w i v .R &v 3. 4. 1. Z

Filed Jan. 12, 1962 FOLLOWER FLUID PLUG CEMENT PLUG CEMENT FIG.

INVENTORSI FIG.

THEIR AGENT United States Patent 3,183,971 PRESTRESSING A PIPE STRING IN A WELL CEMENTING METHOD James W. McEver and William K. Godfrey, Houston, Tex., assignors to Shell Oil Company, New York, N.Y., a corporation of Delaware Filed Jan. 12, 1962, Ser. No. 165,948 3 Claims. (Cl. 166-21) This invention relates to a well cementing method and pertains more particularly to the cementing of pipe or casing strings in oil and gas wells.

After a well is drilled a string of casing or a pipe string is subsequently suspended in the well from the wellhead at the surface and cemented in place. Oftentimes well drilling operations are then continued until a predetermined depth has been reached at which time a second string of well casing is positioned and cemented in a well. Since a well may traverse a Water-bearing formation prior to encountering an oil or gas formation, the casing is cemented in the well in order to shut off or isolate the water-bearing Zone from any oil or gas-producing zone within the well.

A certain percentage of well cementing jobs are not successful resulting in water contamination of an oil or gas zone. The reasons for one well being successfully cemented while another well cementing operation is unsuccessful is seldom precisely known. Whatever the causes may be, the result is that water seeps or flows past the cemented portion of the annulus space outside the well casing either due to poor bonding between the well casing and the cement or between the cement and the adjacent wall of the well. The requirements for a good cementing bond become much greater when it is necessary to stimulate (acidize or fracture treat) the productivity of the oil or gas zone. For example, the channel of communication to the undesirable water zone may initially be so small that it is of little or-no consequence. But after a small amount of stimulating fluid has passed through the channel it becomes sufiicient by large to damage the well.

It is therefore a primary object of the present invention to insure a fluid-tight and/or pressure-resistant bond between the cement and the well casing, and between the cement and the borehole wall.

Another object of the present invention is to provide a method of completing wells wherein a well casing is cemented in the well under conditions which diifer from the normal conditions encountered by a well casing in the well during the production life of the well.

These and other objects of this invention will be understood from the following description of the method of the present invention taken with reference to the drawing, wherein:

FIGURES 1 and 2 are diagrammatic views taken in longitudinal cross section of a well casing positioned with a well during cementing operations.

Pipe or casing strings maybe cemented in a well by any of several methods. A general method is to pump a quantity of fluid from the surface down the string of casing so as to force ahead of it the drilling fluid in the casing and in the annular space outside the casing between the casing and the borehole wall, as the mud in the casing would only tend to contaminate the cement. A bottom cementing plug is then placed in the top of the casing string and forced downwardly therethrough by a stream of cementing material. After a predetermined volume of cement has been pumped into the casing string, a second cementing plug or followup plug is generally inserted into the top of the well casing and forced downwardly therethrough by means of a fluid stream, preferably water, which is pumped down the well casing to force the first plug against a seat at the bottom of the casing and then cause a rupture disc in this plug to fall so that the fluid cementing material following the plug can be forced up the annular space outside the well casing to the desired location where the cementing material is allowed to harden.

During cementing operations, the well casing and the cement bond and formation adjacent thereto are subjected to conditions that are not normally encountered during the production life of the well. For example, the particular casing string being cemented may be normally empty of fluid when the well is produced. However, if the same casing string is full of fluid while the cementing material outside it is hardening, the weight of the fluid within the casing string has a tendency to cause the casing string to expand against the cementing material. This expansion stress to which the well casing is subjected during cementing operations is removed as soon as the casing is emptied of fluid. However, it may be that in emptying the well casing of fluid and allowing the well casing to return to its normal condition, the bond between the outer wall of the well casing and the hardened cementing material may be substantially weakened or even broken so that it is no longer fluid tight. It has been found that this expansion stress can be removed from the well casing by pumping out or otherwise emptying the well casing of any fluid to a depth at least near the level of the cementing material positioned outside the well casing. This fluid within the well casing should be removed before the cementing material outside has solidified. Generally a check valve (or valves), called float shoes or float collars, at bottom of casing string closes the bottom of the well casing to hold the cementing material at its desired level. Another way to eliminate this expansion stress is to displace (pump down) the cementing material with a gas. Subsequently, the gas pressure can be relieved at the surface.

A second condition encountered by the well casing and adjacent formation during cementing operations is that of high temperatures. Considerable heat is given off during the hardening of cement and it is difficult and takes considerable time to dissipate this heat in a confined Zone like a well borehole. The temperature within the well itself may be considerable at the level at which the cementing operation is to take place, thus adding the difiiculty of dissipating the heat in the well. In most oil fields, the temperature of the formation at a predetermined depth may be calculated by a rule of thumb in which one takes the surface temperature and adds to it between one and one and a half degrees for each feet of depth that the well is drilled. It a moderately strong bond strength between casing/ cement and cement/ borehole wall is obtained at a temperature elevated above normal, subsequent cooling will cause contraction that will damage or eliminate the bond. Contrastingly, if the same bond strength is obtained at a temperature that is reduced below normal, subsequent warming will cause expansion that will substantially increase the bond strength, this due to the physical characteristics of the materials involved. Both pipe and the normal cementing material have thermal coeflicients of expansion (or contractions) that are in the order of 1 /2 to 5 times the coeflicient of a rock.

In accordance with the well completion method of the present invention, a string of well casing is first positioned or suspended within a well borehole in a manner well known to the art, after which a quantity of sealing material is deposited in the space between the well casing and the wall of the well either by forcing it down the space, or by first pumping it down the well casing and circulating it up the annular space outside the casing in a manner Well known to the art to a desired location. The sealing material may be any of the well known cement mixtures or cement slurrys or liquid plastic materials which have a sufficiently slow setting time so that they will not set up in the time it takes them to be pumped down a well and into position at the temperatures encountered in the well to be sealed. Latex cement may also be used or any other well cementing combinations. The sealing material or cement slurry is allowed to stand undisturbed for a period sufficient for the material to harden and bond to the outer surface of the well casing and the wall of the borehole thus forming an eflicient water shutoff closure.

For at least a major portion of the material hardening period, the portion of easing opposite the zone to be cemented and/ or the volume of cementing material itself are subjected to and/ or controlled in an abnormal condition or state such that when they are allowed to return to normal the casing will, in effect, be exerting an expansion type force through the cement to the borehole wall, i.e., the conditions affecting the volume of the casing and the cementing material are controlled so that the cementing material hardens when the casing has an abnormally small diameter as compared to its diameter during subsequent periods under normal well conditions for that well at the depth of the cement bond. The final result is a prestressed condition in the casing.

Prestressing of a portion of the well casing or pipe string and the sealing material adjacent thereto may be accomplished by cooling the casing or material adjacent thereto to a temperature below that normally encountered during the normal well operations at the depth of the material. The necessary temperature control can be obtained in numerous ways. For example, a precooled fluid may be circutlated down the well casing and up the annular space outside the casing to cool the casing and adjacent formation prior to introducing the cementing materials. Alternatively and/ or additionally, the cementing materials and displacing materials can be precooled at the surface before introduc ing them into the well. Additionally, a heat-adsorbing material, such as ice, can be spotted in the portion of well casing that is surrounded by the cementing material. If desired, a cooling liquid can be circulated down the well casing and into contact with that portion of the casing opposite the cementing material in a manner well known to the art. In some installations it will be found feasible to simply pump the cementing and displacing materials into place at surface temperature and then swab out the displacing fluid to a level adjacent the cementing material, relieving the casing of forces tending to expand it, and then dropping into the casing pellets of a low-melting solid material having a density greater than that of the displacing fluid. Where it is feasible to use oil or light hydrocarbons as a displacing fluid, such pellets may be ordinary ice; where water is used as the displacing fluid, the pellet can comprise the relatively high density solids which are formed by freezing salt solutions, such as sodium or calcium chloride. Where such low-melting solids are used, it may be desirable to control their effective life by insulating thermally by freezing porous materials, such as sawdust, fullers earth, or the like, impregnated with the liquids.

Where it is feasible to use a light hydrocarbon as a displacing fluid, this fluid can be selected so that its prop erties aid in both reducing the pressure within the casing and absorbing the heat generated during the setting of the cement. This is accomplished by selecting a normally liquid hydrocarbon having a boiling point substantially equal to the formation temperature. When such a hydrocarbon is used as the displacing fluid and the fluid level is lowered to near the depth of the cementing material, or where a water-base displacing fluid is used and then removed and portions of such a hydrocarbon are pumped into the portion of the casing string adjacent to the cementing material, the heat generated by the setting of the cementing material is absorbed by the evaporation of the hydrocarbon.

It has been found that by carrying out cementing operations at reduced temperatures, that there is less shrinkage separation between a cement and a well casing and between the cement and the adjacent formation, say limestone, than there is at high temperatures. It may be seen that by either cooling the well casing and/ or cement during the major portion of the material hardening period, and/ or by removing fluid from inside the casing at a point adjacent the hardening material, that a better bond is formed between the cement and the well casing and between the cement and the formation.

One form of the present invention is illustrated in FIG- URES 1 and -2 of the drawing wherein a well casing head 10 is illustrated as being positioned at the top of a well -11 in which a string of casing 1-2 is suspended, During cementing operations, which are carried out in a manner well known to the art, cement is circulated down the casin g string 12 followed by a cementing plug 13 which in turn is propelled downwardly by a follower fluid 14 under pump pressure. As the cement is forced out of the bottom of the casing string 12, it rises in the annular space 15 outside the casing string. Thus, by the time the plug 13 reaches the bottom of the casing string (FIGURE 2) the cement in the annulus 15 has been forced up to the desired level. In accordance with the present invention the follower fluid 14 may be a cooling fluid which may contain, for example, ice pellets, or the ice pellets 16 (FIGURE 2) may be dumped into the casing string 12 after the plug has reached the bottom of the string. The cooling fluid with or without the ice pellets 16 preferably extends upwardly over the desired interval of the casing string that it is desired to pre-stress.

We claim as our invention:

1. In well operations the method of sealing an elongated tubular member in a well borehole, said method comprising (a) positioning a string of pipe in a well,

isting in a zone of the well to be sealed with a sealing material,

(c) depositing a quantity of sealing material in the space between said pipe string and the wall of the well,

(d) allowing said sealing material to stand undisturbed for a period sufficient for said material to harden and form a closure between the pipe string in the wall of the well,

(2) prestressing the portion of pipe string adjacent said material for at least a major portion of the material hardening period, said prestressing step comprising controlling the temperature of said sealing material so thafdiirin'g the conversion of said sealing material from a fluid to a rigid solid the temperature thereof (b) determining the normal stabilized temperature ex is maintained below the normal temperature of the well zone being sealed.

2. The method of claim 1 wherein the cooling step comprises depositing a heat-absorbing material within said pipe string at a level opposite at least a major portion of Said sealing material outside said pipe string.

3. In well operations the method of sealing an elongated tubular member in a well borehole, said method comprismg (a) positioning a string of pipe in a well,

(b) depositing a quantity of sealing material in the space between said pipe string and the wall of the well at a predetermined level,

(0) allowing said sealing material to stand undisturbed for a period sufi'icient for said material to harden and form a closure between the pipe string and the wall of the well, and

3,183,971 5 6 (d) producing a final prestressed condition in a por- References Cited by the Examiner tion of said pipe string by contracting the portion of UNITED STATES PATENTS the pipe string adjacent the level of at least a portion of the sealing material for at least a major por- 11647003 10/27 Huber 16621 tion of the time during which the material hardens, 1,866,522 7/32 Jackson et a] 16629 X (e) the step of contracting a portion of the pipe string 3,064,436 11/62 Lo?fbourow et 166mm X comprising cooling the portion of pipe string adjacent 3,097,691 7/63 Smlth 166'28 said sealing material to a temperature below that normally encountered during normal operations at the BENJAMIN HERSH Examiner depth of the material. 10 BENJAMIN BENDETT, Examiner. 

3. IN WELL OPERATIONS THE METHOD OF SEALING AN ELONGATED TUBULAR MEMBER IN A WELL BOREHOLE, SAID METHOD COMPRISING (A) POSITIONING A STRING OF PIPE IN A WELL, (B) DEPOSITING A QUANTITY OF SEALING MATERIAL IN THE SPACE BETWEEN SAID PIPE STRING AND THE WALL OF TH WELL AT A PREDETERMINED LEVEL, (C) ALLOWING SAID SELAING MATERIAL TO STAND UNDISTURBED FOR A PERIOD SUFFICIENT FOR SAID MATERIAL TO HARDEN AND FORM A CLOSURE BETWEEN THE PIPE STRING AND THE WALL OF THE WELL, AND (D) PRODUCING A FINAL PRESTRESSED CONDITION IN A PORTION OF SAID PIPE STRING BY CONTRACTING THE PORTION OF THE PIPE STRING ADJACENT THE LEVEL OF AT LEAST A PORTION OF THE SEALING MATERIAL FOR AT LEAST A MAJOR PORTION OF THE TIME DURING WHICH THE MATERIAL HARDENS, (E) THE STEPS OF CONTRACTING A PORTION OF THE PIPE STRING COMPRISING COOLING THE PORTION OF PIPE STRING ADJACENT SAID SEALLING MATERIAL TO A TEMPERATURE BELOW THAT NORMALLY ENCOUNTERED DURING NORMAL OPERATIONS AT THE DEPTH OF THE MATERIAL. 